1. Field of the Invention
The present invention pertains to asynchronous communication systems and methods for communicating information over an Integrated Services Digital Network (ISDN) network. In particular, the present invention pertains to establishing over an ISDN D channel an asynchronous communication link between a source terminal and a destination terminal via an ISDN switch, where the established communications link has communications attributes that are favorable to both the source terminal and the destination terminal.
2. Discussion of the Background
Conventional facsimile devices communicate over the Public Switch Telephone Network (PSTN) using analog signals that are transmitted over conventional telephone lines. The source terminal (e.g., a facsimile device, computer with scanner and modem facilities, or another device that transmits and/or receives data) converts digital scanned information into a corresponding analog signal so the same may be sent over the PSTN telephone line to the destination terminal. In turn, the source terminal receives the analog information and converts the analog information back into digital signals which form the basis of an image to be printed, perhaps on facsimile paper.
The Integrated Services Digital Network (ISDN) is evolving into a next generation worldwide public telecommunications network that will replace existing public switch telephone networks and provide a variety of services that are not offered by the PSTN. ISDN will allow the transmission of various types of data between various types of ISDN terminal equipment (TE).
A portion of the ISDN link between a source terminal and a central office, which has a switch facility, is referred to as a xe2x80x9cdigital pipexe2x80x9d. A capacity of the pipe is generally discussed in terms of separate channels. In particular, a xe2x80x9cbasic accessxe2x80x9d digital pipe includes two B channels (basic channels) which support 64 kbps signaling, and a D channel at 16 kbps. While the total bit rate of these three channels is 144 kbps, framing, synchronization and other overhead bits bring the total bit rate of a basic access link to 192 kbps. However, the B channels serve as separate communication channels such that the maximum data capacity, as view by the user, is 64 kbps per channel, not 192 kbps.
The function served by the D channel, is twofold. First, the D channel is used to establish and maintain signaling between the user""s equipment and the ISDN""s switch (operated by the telephone company). Thus, the D channel carries signaling information such as that required for dialing the telephone number of the destination terminal and making the connection between the source terminal and the destination terminal. A more complete description of narrowband and broadband ISDN, as well as ISDN terminal equipment, protocols, data rates, etc. is provided in the literature, for example in Stallings, W., xe2x80x9cData and Computer Communicationsxe2x80x9d, 5th Edition, Prentice Hall, 1997, pp 740-769 (hereinafter xe2x80x9cStallingsxe2x80x9d) the contents of which is incorporated herein by reference.
FIG. 1 shows a conventional ISDN system where a source facsimile 10 at a source facility 1 communicates via an ISDN switch 22 to a destination facsimile 16 (or other type of destination terminal, such as a computer, ISDN equipped photocopier, etc.) in a destination facility 2. The source facsimile 10 communicates via a terminal adapter 10A, which is shown as an internal device, although a separate external terminal adapter may be used as well. The terminal adapter 10A provides a protocol (physical layer and intermediate layer) conversion function for converting signal protocol such as V.35, RS-232, Universal Serial Bus (USB), IEEE 1394 (FireWire), etc. to an ISDN compliant protocol over a 4-wire interface.
The source facsimile 10, terminal adapter 10A, and network termination (NT1) 14, are included at the users"" source facility 1. The NT114 connects, via a two-wire line 15, to a switching module 26 located at the ISDN switch 22. Alternatively, a second network termination (NT2) may be used at the source facility 1 between NT1 and the terminal adapter to provide a switching and concentration function, such as with a digital private branch exchange (PBX). Likewise, the NT1 may be replaced with a NT12 that performs the functions of both the NT1 and NT2.
At the ISDN switch 22, the switching module 26 connects to a processor 24 and another switch module 28 via a bus 27, which allows digital commands and data to be passed between the respective switching modules 26 and 28, and processor 24.
The equipment at the destination facility 2 may or may not be exactly similar to that of the source facilities 1. It is assumed, however, that the source facsimile 10 and the destination facsimile 16 have no prior knowledge of the other""s communication attributes. In the system shown at FIG. 1, the destination facility 2 includes the destination facsimile 16 having a terminal adapter 16A incorporated therein, which connects to another NT120 as shown. The NT120 connects to the switching module 28 in the ISDN switch 22, via another two-wire line 17 as shown.
ISDN communications is based on a seven layer protocol stack, as explained in reference to FIG. A.5 of Stallings, for example. Control signaling is accomplished between the respective user-network interface and occurs at a third layer of the protocol stack (i.e., the xe2x80x9cnetworkxe2x80x9d layer) and is named I.451/Q.931. Thus, establishing and maintaining control signaling for a communication link established between the source facility 1 and a destination ISDN facility 2 through the D channel, and in particular, the ISDN network layer, data link layer and physical layer.
A link access protocol (LAPD) D channel is defined for establishing particular LAPD frames that are exchanged between the subscriber equipment (either at the source facility 1 or at the destination facility 2) and the ISDN switch 22. The call control protocol I.451/Q.931 is used on the D channel to establish, maintain and terminate connections on B channels.
FIG. 2 illustrates the signaling sequence between the source facility 1 and the ISDN switch 22. In order to establish a B channel connection between the source facility 1 and the destination facility 2, an initial communication link must be established on the D channel between the source facility 1 and the destination facility 2. To this end, a series of messages is sent back and forth between the source facilities 1 and the ISDN switch 22. This communication between the source facilities 1 and ISDN switch 22 occurs on a continuing basis on the D channel, while communications are maintained between the source facilities 1 and destination facilities 2 on the B channel. As shown in FIG. 2, several different messages are sent between the source facilities 1 and ISDN switch 22 while the D channel is maintained.
The direction of the arrows in FIG. 2, indicates a direction of communication between the source facilities 1 and the ISDN switch 22. The process for establishing a connection is initiated by the source facilities 1 by first sending a setup message. Particular features of the setup message will be discussed with respect to FIG. 3, however the purpose of the setup message is to provide general information regarding the request to connect to the ISDN switch 22. Next, the ISDN switch 22 responds with a call proceeding message that indicates that call establishment has been initiated. Subsequently, the ISDN switch 22, sends a connect message that indicates call acceptance by the source facilities 1. The source facilities 1 then sends a connect acknowledge signal that indicates the user has been awarded the call. When the user wishes to disconnect a call, the user sends a disconnect message via the source facilities 1 to the ISDN switch 22, requesting for connection clearing. In response, a release message is sent from the ISDN switch 22, indicating the intent to release the channel and call reference. In response the source facilities 1 issues a release complete message, indicating that the release of the channel and the call reference. Subsequently, the call and information flow through the B channel is terminated.
FIG. 3 shows the structure of a conventional ISDN D channel setup message. The setup message includes respective LAPD frames (e.g., 501, 503 . . . ) of different sizes (measured in octets). The message includes a flag frame 501 that is one octet in length, followed by a service access point identifier (SAPI) frame 503 having a command/response bit (CR) and address field extension bit (0). The SAPI frame 503 is joined with the terminal end point identifier (TEI) frame 505, each of which are one octet in length. A control frame 507, is one or two octets in length, and is followed by an information frame 509, which has a variable length between 0 and 128 octets. A frame check sequence frame 511 follows and occupies two octets in length. An end frame 513 serves as an end of setup message flag.
The SAPI frame 503 includes a first subfield xe2x80x9cSAPIxe2x80x9d, that identifies a protocol layer-3 user, as well as subframes C/R and 0, that are used as a predetermined formatting feature of SAPI. The terminal end point identifier frame 505, is used to provide a unique terminal end point identifier that is used to identify the user""s equipment. The control frame 507 defines the type of frame format that will be employed such as an information frame, supervisory frame, and unnumbered frame for example. The information frame 509, includes a variable number of octets varying from 0 to 128 and contains respective subfields that contain any sequence of bits that form an integral number of octets.
Thus, when a user wishes to send data to a destination, information in the information field is passed directly to the destination user without the ISDN switch deciphering the contents of the information. Following the information field 509, the frame check sequence 511 is included and forms an error-detection function by calculating a code from the remaining bits of the frame, exclusive of the flags. The normal code is a cyclical redundancy check code. Finally, the end flag frame 513, includes a specific code indicating the end of the setup message.
As identified by the present inventor, a limitation with the conventional ISDN setup architecture is that there is no suitable approach for coordinating, optimally, the communication attributes of the source terminal and the destination terminal on the D channel prior to allocating or using the B channels. Moreover, the ISDN switch, while being able to relay information directly to a destination terminal, passes this data on directly, without interpreting the data or the contents therein.
Furthermore, the conventional ISDN terminals and ISDN switch are not configured to perform a communication resource allocation and optimization process using the D channel so that while establishing the request for a particular B channel, the ultimate resources assigned by the ISDN switch to the source terminal and destination terminal are optimum based on the attributes of the source terminal and the destination terminal.
The present inventor also determined that the conventional approach for communicating over the ISDN channel is not well suited for older facsimile machines and other devices that send facsimile messages, perhaps by way of asynchronous communication protocols.
Furthermore, the present inventor determined that in order to accommodate different vintage source and destination terminals, there may be the possibility of manually, or alternatively automatically, configuring the communication parameters between the source terminal and the destination terminal so that a quick communication link may be established through the ISDN switch. A typical messaging sequence of a G3 facsimile machine that communicates to a called facsimile machine by way of a telephone network is shown in FIG. 13. The different acronyms used in FIG. 13 correspond with the acronyms and asynchronous protocol symbols shown in FIG. 22. FIG. 14 shows a communication sequence between a calling facsimile device and a called facsimile device on a telephone line, after which the facsimile devices return to an idle mode. FIGS. 15A-15C show respective frame structures used to specify non-standard facilities (NSF), the non-standard facilities set up (NSS) and the non-standard facilities command (NSC). Thus asynchronous communications may use these frames, as presently recognized, in an asynchronous communication session. FIGS. 16A through 16G show conventional communications sessions between a facsimile device (calling device) and a called device. The communications sessions shown in FIG. 16A relates to calling condition initiated from an idle line condition. The communication session in FIG. 16B relates to a situation where the non-standard facilities function is requested by the called device. FIG. 16C shows a communications session where a multi-page signal is sent by the calling device, informing the called device that several pages will be sent, where the set-up data is the same between the respective pages. FIG. 16D shows a communications session for a polling operation. FIG. 16E shows a communications session for a turn around polling operation, where FIGS. 16F and 16G show an end to the calling session between the calling device and the called device. FIG. 17 shows typical communications sessions for a fax PC link command exchange sequence. In particular, FIG. 17A shows a situation where a command from the called device causes consecutive errors. FIG. 17B shows a situation where the calling device causes consecutive errors in the transmission to the called device. FIG. 17C shows a situation where the first xe2x80x9cconfirmation to receivexe2x80x9d signal is received in error by the calling device, and the technique for overcoming that particular error sequence.
The different protocols and signaling sequences in FIGS. 13-14 and 16A-16G and 17A-17c, are conventional approaches for communicating between a calling device and a called device, when conveying information between the two, over a conventional telephone line.
As presently recognized, limitations with the conventional communication sessions as discussed above, are that the communication terminals themselves may be better served by a communication channel that is adapted for the communication of information most efficiently between the calling terminal and the called terminal. However, conventional techniques do not enable the ability for the two terminals to agree upon a set of communication protocol features that will enable the most efficient communication between the two devices. It is presently recognized that if such a coordination mechanism were made available, it would be possible to employ older-style facsimile devices and other devices that send facsimile messages in a most efficient manner over an ISDN line via an ISDN switch.
Accordingly, one object of this invention is to provide a novel method, apparatus and system for establishing optimized communication conditions between an ISDN source terminal and an ISDN destination terminal that overcomes the above-mentioned limitations of existing methods, apparatuses and systems.
It is a further object of the present invention to provide a method, and apparatus, that employs a D channel of an ISDN basic service, for providing information to an ISDN switch regarding the communication attributes of the ISDN source terminal for establishing manually or automatically an asynchronous communication link between the source terminal and destination terminal.
Another object of the present invention is to provide a modified ISDN switch that is capable of identifying the communication attributes of the source terminal, and relaying a request message to the destination terminal, so that the ISDN switch can provide an optimal match of communications resources, based on the attributes of the source terminal and the destination terminal.
It is another object of the present invention to provide a computer-based apparatus and method that may be employed in ISDN user terminals, and related methods and processes provided at an ISDN switch, for identifying and coordinating attributes between a source terminal and destination terminal only on a D channel of an ISDN basic service.
It is a further object of the present invention to establish this optimization procedure on the ISDN D channel, before the B channel communication link has been established between the source terminal and the destination terminal.
These and other objects are achieved with the inventive method, apparatus and system, that forms a setup message at a source ISDN terminal where the setup message includes the communication attributes of the source terminal. The ISDN source terminal, relays the setup message with the attribute information to an ISDN switch, where the ISDN switch implements a central resource coordination mechanism that extracts the attribute information from the source terminal, and initiates communications over a D channel with a destination terminal so as to ascertain the communication attributes of the destination terminal. In light of the communication attributes of the source terminal and destination terminal, the ISDN switch and central resource coordination mechanism contained therein, compares and allocates ISDN communication resources for optimally using the attributes of the source terminal and destination terminal prior to the allocation of B channels, so that a most efficient, and highest capacity communication link may be established between the source terminal and destination terminal using the assigned B channels.
The present invention also includes a feature of permitting the source terminal and destination terminal to communicate over an asynchronous communication protocol, where attributes associated with the facsimile message (such as image resolution, number of black/white shades of the image, and number of shades of color features for the image to be transferred) are specifiable, either automatically through a terminal adapter, or manually, as requested by a user of the source terminal, for example. Manually establishing these particular parameters, enables a more rapid set up between the source terminal and destination terminal.
The present invention will require an addition to ISDN switch facilities that is not currently present. In particular, the ISDN switch processor, and computer based resources, must be modified so as to interpret, and make inquiries about the communication attributes of the source terminal and destination terminal, and subsequently allocate communication resources between the terminals so that optimal communication conditions may be established. In this way, the coordination between the resources is accomplished using the D channel, without requiring the additional time and resources at the source terminal and destination terminal using the B channel, so that calls may be established more quickly, and handshaking process between the source channel and destination terminal may be streamlined, for ultimately a lower cost service to the user.